Fixes ICE when using reexported unit-like structs

Fixes that unit-like structs cannot be used if they are reexported and
used in another crate. The compiler fails with an ICE, because unit-like
structs are exported as DefFn and the expression `UnitStruct` is
interpreted as function pointer instead of a call to the constructor.

To resolve this ambiguity tuple-like struct constructors are now exported
as CtorFn. When `rustc::metadata::decoder` finds a CtorFn it sets a new
flag `is_ctor` in DefFn to true.

Relevant changes are in `rustc::metadata::{encoder, decoder}` and in
`rustc::middle::ty`.

Closes #12660 and #16973.